Donald Trump is now the top Republican in the race for president — but the top Republican in Congress isn't prepared to get behind him.

"To be perfectly candid with you Jake, I'm just not ready to do that at this point," House Speaker Paul Ryan told CNN's Jake Tapper Thursday when asked about endorsing Trump. "I'm not there right now. And I hope to, though, and I want to."

Ryan, who is also chairman of the Republican National Convention, stressed that "what's required is that we unify this party... I don't want to underplay what [Trump] accomplished. He needs to be congratulated for an enormous accomplishment... But he also inherited something very special... This is the party of Lincoln, of Reagan, of Jack Kemp."

The speaker attributed at least part of his reticence to having been 2012 GOP nominee Mitt Romney's running mate and Trump's relentless criticism of their campaign.

"It's because I think of part of the last campaign," Ryan said during an appearance from Burlington, Wisconsin. "I don't want to go back and roll the tape; I was clear and outspoken on a number of occasions and I think that he did [and] said the wrong thing."

(Romney, of course, has completely unloaded on Trump as a con artist at best, as have other prominent Republican officials and media outlets.)

Ryan also said that after this cycle's slash-and-burn campaigning, Trump has a trek ahead of him to prove that he can be a unifying GOP standard bearer: "Let me say it this way: Republicans have been watching them go after each other for six months."

It also remains to be seen, Ryan said, whether Trump can convince the party he shares their conservative views by "actually taking the principles that we all believe in, showing that there's a dedication to those, and running a principled campaign that Republicans can be proud about and that can actually appeal to a majority of Americans."

Ryan was himself at one point talked up as a "white knight" candidate the GOP establishment could advance if Republicans didn't settle on a nominee before a contested July convention in Cleveland.

He continued to rule that out as he refused to endorse Trump — something Senate Majority Leader Mitch McConnell has said he would do.

Now that Trump has toppled his final primary foes, he's officially the presumptive nominee.

As evidenced by the hesitance of Ryan and other top GOP leaders — up to and including the last two Republican presidents — to come out in favor of Trump's candidacy, however, that doesn't mean The Donald's already won.